Method of forming hard, abrasionresistant coatings on aluminum and aluminum alloys



Patented Oct. 26, 1954 METHOD OF FORMING HARD, ABRA-SION- R-ESISTAN T COATINGS N ALUMINUM AND ALUMINUM ALLOYS Charles F. Burrows, Baltimore, Md., assignor, by mesne assignments, to Aluminumcomp'any of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application April 22, 1950, Serial No. 157,640

4 Claims. 1

This invention relates to the production of hard, abrasion-resistant coatings an aluminum and aluminum alloys. More particularly, the invention relates to a method for the production of coatings on aluminum and aluminum alloys by anodic oxidation in such manner that the resulting coatings are characterized by an unusually high degree of abrasion resistance as compared with coatings produced on aluminum and aluminum alloys by previously known methods. The invention also relates to aluminum and aluminum alloy articles that have been provided with the aforesaid hard abrasion-resistant coatings.

Thus, one of the principal objects of the present invention is to provide a new and improved method for producing coatings upon aluminum and aluminum alloys that are characterized by a degree of hardness, abrasion resistance and corrosion resistance that, so far as I am aware, is completely unknown to prior art workers in this field. A further object of the invention is to provide a method for producing coatings on aluminum and aluminum alloys by anodic oxidation that is characterized by the ability to produce coatings of unusual thickness on the base metal.

The manner in which these and other objects and features of the invention are attained will appear more fully from the following description thereof, in which reference is made to typical and preferred procedures in order to indicate more fully the nature of the invention, but without intending in any way to limit the scope of the invention thereby.

It has already been proposed to coat aluminum and aluminum alloy articles my anodic oxidation in an electrolyte comprising aqueous sulfuric acid; for example, see Bengstons U. S. Patents Nos. 1,869,041, 1,869,042, 1,891,703, Tosteruds U. S. Patent No. 1,900,472 and Works U. S. Patent No. 1,965,682. In accordance with this method, the aluminum or aluminum alloy article to be coated is made the anode in an electrolytic cell having a lead cathode and an aqueous sulfuric acid electrolytic bath, and the coating is produced upon the article by passing a direct current having suitable voltage and current density characteristics through the system, all as well known to those skilled in the art. These coatings are comprised principally of aluminum oxide. While the patent literature occasionally refers to the production of a coating having a thickness of "0.0015 inch or more (for example, see Bengston, 1,869,041, at page 3, lines 110-112), so far as I am aware it has not been found possible to produce coatings appreciably greater than 0.0015" in thickness in actual commercial practice. Furthermore, While the coatings produced by prior art methods in the aforesaid manner are characterized by a certain degree of abrasion resistance, there are many applications for which it would be very desirable if one could obtain aluminum oxide coatings on aluminum and aluminum alloy articles that were characterized by substantially increased abrasion resistance.

According to the present invention, it has been found that coatings of substantially greater thicknesses and of much higher abrasion resistance may be produced by anodic oxidation of aluminum and aluminum alloy articles if the conditions under which the coatings are produced are modified in respects to be indicated hereinafter.

Before indicating the precise manner in which the present invention is to be practiced, it will be desirable to describe briefly the prior art procedures that have been worked out for the anodic oxidation of aluminum and its alloys.

According to prior practices, the anodic oxidation is generally carried out in an aqueous sulfuric acid electrolyte, the concentration of which may vary over a wide range with respect to the sulfuric acid. Sulfuric acid concentrations of from 1 to 70% have been suggested in the prior 'terature. According to accepted prior art practice, electrolyte temperatures varying from room temperature and higher have been suggested. For example, in Bengston Patent No. 1,869,041, at page 3, lines 40-42, it is stated that: It is best to keep the temperature between to 30, preferably at C. The range set forth in this passage corresponds to 68 F. to 86 F. A similar statement appears in Bengston Patent No. 1,869,042 at page 2, lines 44-46, and in the Tosterud Patent No. 1,900,472 at page 2, lines 22-24.

I have discovered, however, that if the temperature of the electrolytic bath comprising aqueous sulfuric acid is maintained at all times during the coating operation at a value not exceeding 40 F. and preferably at a value of about F., there occurs what appears to be a fundamental change in the modus operandi by which one is enabled to produce coatings that are much thicker and more highly abrasion-resistant.

The coatings according to the present invention are produced by immersing the aluminum or aluminum alloy articles to be coated in an aqueous electrolyte, preferably comprising an aqueous solution of sulfuric acid, that is maintained at a temperature of not more than 40 F., and preferably about 30 F., during the entire coating procedure, the coating being produced by passing a suitable electric current through the articles to be coated, as anodes. The necessary low temperature is maintained by suitable refrigeration of the electrolyte, and preferably but not necessarily by providing means for recirculating the electrolyte through an outside cooler since in that manner the electrolyte is simultaneously subjected to vigorous agitation which assists in maintaining the necessary low temperature uniformly throughout the entire body of electrolyte. Whatever method of cooling is employed, care should be taken to insure efficient agitation of the electrolyte with consequent substantial uniformity both of temperature and composition throughout the electrolyte. While the electrolyte temperature is maintained at not more than 40 F., and preferably at about 30 F. throughout the entire coating operation, the temperature may be maintained at lower values if desired, for example, at temperatures as low as about 4 Fahrenheit degrees above the freezing point of the particular electrolyte chosen for the coating operation. As a general rule it will be found that a maximum electrolyte temperature of not more than 35 F. will give satisfactory results for most purposes, although as indicated above a maximum of 40 F. may be resorted to without sacrificing all of the important advantages of the present invention. In all cases, however, it is quite important to provide means for insuring thorough agitation and mixing of the electrolyte in order to insure substantially uniform temperature and composition throughout the entire body of electrolyte.

The remaining conditions of treatment during the coating operation may be in substantial accordance with those heretofore known to the prior art, if desired. No particular pre-treatment of the aluminum or aluminum alloy article to be coated is necessary except of course the conventional one of treating the articles, if necessary, with a suitable cleansing bath in order to make sure that they are clean and free from grease and dirt or dust.

The anodizing operation may be conveniently carried out in conventional electrolytic apparatus by immersing the aluminum or aluminum alloy articles to be coated in a lead lined tank in which the lead lining is employed as the cathode.

The electrolyte may be an aqueous solution of sulfuric acid in which the acid concentration varies from about 5 to 70% H2804. Acid concentrations of from about 5 to 25% have been found to be particularly satisfactory, with about H2804 representing an optimum value for most coating operations.

In addition, one may employ coating bath additives in conjunction with the aqueous sulfuric acid electrolyte. A number of these known to the prior art are suitable, such as oxalic and other organic acids as disclosed in Works U. S. Patent No. 1,965,682, glycerin, and the like.

The current characteristics employed for the coating operation may be varied over wide ranges. I have found that it is desirable to employ current densities within the range of about 10 to 50 amperes per square foot, with current densities of about to amperes per square foot representing optimum conditions. The voltage may vary from 25 to volts at the beginning of the operation, generally increasing during the course of the operation to a maximum of the order of 40 to 60 volts. It Will be appreciated that a desirable manner of regulating the current density is to increase the voltage during the course of the coating operation so as to maintain the desired current density. While direct current is preferred, alternating current may be employed if desired, although in such case the electrical efiiciency of the coating operation will be decreased by approximately one-half since the coating will be then formed during only one-half of each current cycle.

The time during which the coating operation is carried out may vary over rather wide ranges, it having been found that coating times of from one-half to four hours are satisfactory for most purposes. A particularly desirable time is approximately ninety minutes when operating at current densities of approximately 20 to 25 amperes per square foot, in which case the thickness of the anodic film produced thereby is of the order of about 0.002".

The coatings do not require any after-treatment beyond a plain water rinse in order to remove residual electrolyte from the surface thereof. However, the coatings may be dyed by standard dyeing techniques well known in this particular art, and in this manner they may be colored black, blue, red, yellow, etc., as desired.

In cases where it may be desired to impart maximum corrosion resistance to the coating, this may be done by sealing the coating by dipping it in hot water maintained at a temperature from about F. to the boiling point for a period of about ten minutes, which apparently has the effect of changing the coating from an amorphous aluminum oxide to the monohydrate form. It should be noted, however, that this treatment decreases the abrasion resistance of the coating, and hence is not recommended where maximum abrasion resistance of the coating is required.

The coatings as obtained by the process of the present invention are colored from light gray to almost black, depending in part at least upon the particular aluminum or aluminum alloy composition undergoing treatment.

The coatings obtained in accordance with this invention vary in thickness depending upon the conditions under which they are produced, and especially in accordance with the coating times. The following table shows how the thickness of the coating varies with time, under preferred conditions of current density (20-25 amperes per square foot) and electrolyte temperature (30 F):

TABLE I Thickness,

The coatings may be applied to articles formed of pure or substantially pure aluminum, or to articles formed of aluminum alloys wherein aluminum is the predominant constituent. Such alloys should generally have an aluminum content of about 85% or more, although it may be noted that copper, if present, should not be present to the extent of more than about 5%. Examples of specific alloys which, in addition to pure or substantially pure aluminum, may be coated in accordance with the present invention with desirable results are as follows: 143 alloy having a composition of about 4.4% copper, 0.8% silicon, 0.8% manganese, 0.4% magnesium, balance aluminum; 24s alloy having a composition of about 4.5% copper, 1.5% magnesium, 0.6% manganese, balance aluminum; 61S alloy having a composition of about 1% magnesium, 0.6% silicon, 0.25% copper, 0.25% chromium, balance aluminum; and 75S alloy having a composition of about 5.5% zinc, 2.5% magnesuim, 1.5% copper, 0.3% chromium, 0.2% manganese, balance aluminum.

The process of the present invention is particularly well adapted for use in connection with the treatment of aluminum and aluminum alloy parts having certain specialized functions where a thick, abrasion-resistant coating is essential. These uses may be said to fall into three main categories, as follows: (1) the treatment of aluminum and aluminum alloy parts intended for use in place of steel parts as a Weight-saving measure; (2) the replacement of aluminumcoated parts in environments where ordinary aluminum parts, either uncoted or coated by prior art methods, wear out so rapidly as to render their use unfeasible economically; and (3) so-called nonseizure applications where two or more aluminum parts that normally are employed in relatively tightly cooperative relation with each other tend to seize or freeze upon each other during use, with resultant jamming of the mechanism of which they are a part.

Specific examples of applications or" aluminum or aluminum alloy parts or articles coated according to the present invention, and tending to fall in one or more of the three main classes mentioned in the preceding paragraph, are threaded aluminum parts; bearing surfaces, both sliding and rolling, and including wear plates, swivel joints and friction locks; gears, and particularly the load-carrying tooth portions thereof; prosthetic devices, with particular reference to the load-carrying joints of leg braces in this class of devices; rotor blades, and especially the leading edges of helicopter blades the tips of which frequently travel at supersoni speeds; ball-bearing raceways; intricate structural shapes including tubular shapes in which it has been dinicult heretofore to obtain the necessary throwing power during the coating operation; and aeronautical uses in general where weight-saving combined with a relatively high degree of abrasion resistance are desiderata.

Aluminum and aluminum alloy articles provided with the aluminum-oxide or aluminumoXide-containing coatings of the present invention are also characterized in that they are relatively good heat insulators.

The coatings of the present invention also offer excellent resistance to atmospheric and salt spray corrosion. Thus test panels provided with 0.022" thick films have shown substantially no corrosion after 14 months continuous exposure to the atmosphere while spraying with a 20% NaCl solution.

The hard coatings of the present invention are non-conductors of electricity. Anodic films produced on aluminum by standard prior art procedures displayed voltage breakdown values of approximately 340 volts, as compared to a range of 550 to 3700 volts for the coatings of the present invention depending upon their thicknesses.

It may be also pointed out that aluminum or aluminum alloy parts which are worn down may be reworked or rebuilt by the application of a relatively thick coating produced in accordance with the present invention. For example, it is possible by this method to rebuild or rework a gear or bearing by building up as much as 0.003" or more on a side or 0.006" on a diameter.

Aluminum or aluminum alloy parts that have been coated in accordance with the present invention may be mechanically worked to some extent but in such cases care must be taken that the degree of working is not so great as to damage the coating which does not possess any great degree of ductility. Under certain circumstances, the coating of the present invention tends to flake off under compression although it is fairly stable under conditions of mild tension. Generally speaking, it is preferable to form the aluminum or aluminum alloy into the shape of the desired article and then subject the finished article to the coating operation, rather than conversely, in View of the fact that the coating of the present invention is approximately file-hard.

In certain applications iIlVQlVil'l aluminum or aluminum alloy parts coated in accordance with the present invention, and particularly where relatively heavy gearing pressures are involved, it is possible further to enhance the resistance to sliding wear of the arts so coat-ed by employing in conjunction h the coating 2. suitable lubricant material. nxamples of good lubricants for under such circumstances are water, graphite greases, and molybdenum disulfide.

As indicative of the increased resistance to abrasion that is characteristic of aluminum and aluminum alloy articles when coated according to the present invention, tests have been conducted with standard test panels of sheet alumi num or aluminum alloy four inche square and about 0.102 thick on a standard ber ahra sion-testing machine, as well as by testing. As is well known to those lled in the art, the Taber testing machine is arranged to bring to bear, with a p1 loading, upon a test panel mounted eon two standard abrasive wheels which caused to track on the surface of the test panel with a combined rotating and dragging action similar to that of the well-known edge-runner mill. The number of revolutions or cycles of the abrasive wheels which a given test panel will withstand under these predetermined test conditions before the coating is worn through gives a rather accurate indication of the abrasion resistance of the coating. It may be mentioned in passing that the particular Taber abrasion testing machine employed for testing purposes herein was provided with two CS-1*? rubber bonded abrasive wheels and was operated under a standard 1,000 gram loading during all tests.

I have found that aluminum test panels anodically coated according to the bes-u known prior are methods wear through after approximately 26,000 cycles on the Taber testing machine, whereas similar test panels that have b -n coated in accordance with the present invention consistently gave readings ranging as hi h as 690,000 cycles, depending upon the thickness of the coating. Thus, the following table strikingly illustrates these comparative results:

pted and etermined (The difference between the best known prior art method and the present method mentioned in Table II resides in the electrolyte temperature during the coating operation, which was approximately ordinary room temperature (about 70 F.) in the former and about 30 F. in the latter.) As shown by the data above, not only is the method of the present invention capable of producing hard coatings of much greater thicknesses than are obtainable by prior art methods, but also when the coating operation is terminated while the coating is of about the same thickness as prior art coatings the resulting coating is characterized by a much greater abrasion resistance. Thus, having reference to the data for 60 minutes coating time appearing in Table II, it will be observed that for the respective coatings of the same order of thickness, the coating of the present invention outwore the prior art coating by a factor of nearly 6.

According to the gritblast test referred to above, a mass of grit particles of graded size is directed against the coated panel undergoing test by means of a stream of air flowing at a predetermined constant pressure. The total weight of grit employed from the time the test begins to the time the coating wears through under the impact of the grit particles may also be regarded as a good measure of its resistance to abrasion. Under some circumstances this test may be desirable in that it gives a clearer indication of the resistance of the coating to impact stresses. It has been found that test panels subjected to this test showed in every case very substantial improvements in results when they were provided with a coating produced in accordance with the present invention than when they were provided with a coating in accordance with the best known prior art methods.

In order to indicate still more fully the nature of the present invention, the following examples of typical procedures are set forth, it being understood that these examples are presented as illustrative only and that they are not intended to limit the scope of the invention:

Example I A pneumatic piston for use in an aircraft bomb release mechanism, and made of 24S aluminum alloy, was provided with a 0.002" coating by means of anodic oxidation in accordance with the present invention under the following coating conditions:

Direct current at a current density of about 20 amperes per square foot Voltage at the beginning of the coating operation of approximately 26 volts, and increasing thereafter to maintain the desired current density 90 minutes coating time H2504 electrolyte maintained at 30 F., with 8 agitation throughout the entire coating opera-'- tion.

The pneumatic piston thus coated was tested under actual load conditions and was operated through 6,100 cycles, far in excess of the service life required for this member without appreciable wear. The ability to use such a hard-coated aluminum piston resulted in a weight saving of eight pounds per airplane, as compared with the chrome-plated, steel piston that otherwise would have been required for this mechanism.

Example II Both the stirrup and the pivot of a prosthetic leg brace ankle joint were coated in accordance with the present invention. The stirrup was made of 14S alloy and the pivot was made of S alloy. The coating conditions and the thickness of the film were substantially the same as those mentioned in Example I above.

It was found that the leg brace ankle joint treated in this fashion showed a wear measured in the direction of load, of less than 0.001" after 2,158,000 cycles of operation, as compared to a wear of 0.048" after 3,000,000 cycles for similar leg brace parts made of otherwise similar aluminum alloys coated by the previously known sulphuric acid anodic coating process.

Example III Test panels of 75S aluminum alloy were provided with a coating 0.006 in thickness under coating conditions similar to those set forth in Example I except that the coating operation was extended for a total period of four hours. The thus-coated panels were found to withstand 690,000 cycles on the Taber abrasion tester, as contrasted with a value of 26,000 cycles which is the best value that had been obtained with prior art coatings on the same alloy, i. e., commercially coated test panels.

Example IV The tooth portions of an aluminum worm gear were coated to a thickness of 0.002" in accordance with the present invention, the coating extending slightly below the dedendum circle of the gear by masking the remainder of the gear prior to the coating operation in accordance with conventional masking procedures. The coated gear was then driven by a meshing steel worm under moderate loading conditions corresponding to the actual conditions that would obtain in its intended application. At the end of over 35,000 revolutions of the coated gear, corresponding to 445,500 revolutions of the meshing worm (which represented substantially 18 times the anticipated required service life) the parts were disassembled and inspected. While the steel worm showed definite signs of wear, the oxide-coated tooth surfaces of the aluminum worm gear showed none. The substitution of this aluminum gear for the manganese bronze gear which was formerly required represented a weight saving of approximately 2 pounds per unit.

Example V In another application, a 758T aluminum alloy ball bearing ring having its race coated with a 0.002" aluminum oxide coating by the method of the present invention, and provided with steel balls, permitted a weight saving of 40 pounds per unit. This ball bearing operated with satisfactory wear resistance on the race under the relatively moderate loading conditions consistent with the nature of the base metal.

While various specific examples of preferred procedure embodying the present invention have been described above, it will be apparent that many changes and modifications may be made in those methods of procedure without departing from the spirit of the invention. It should, therefore, be understood that the examples cited and the methods of procedure set forth above are intended to be illustrative only and are not intended to limit the scope of the invention.

What is claimed is:

1. A method of producing a hard, abrasion-resistant anodic coating on the surface of a predominantly aluminum or aluminum alloy article by making the article to be coated an anode in an electrolytic cell containing an aqueous acidic coating-forming electrolyte comprising predominantly an aqueous solution of sulfuric acid and passing an electric current through the electrolytic cell, including the anode, while maintaining the electrolyte at a temperature below 35 F. during the entire coating operation.

2. A method of producing on the surface of a predominantly aluminum article an abrasion-resistant aluminum oxide anodic coating by making the article to be coated an anode in an electrolytic cell containing an aqueous acidic coating-forming electrolyte comprising predominantly aqueous sulfuric acid and passing an electric current through the electrolytic cell, including the anode, While maintaining the electrolyte at a temperature between about 4 Fahrenheit degrees above its freezing point and 35 F. during the entire coating operation.

3. A method of producing on the surface of a predominantly aluminum article an abrasion-resistant aluminum oxide anodic coating by making the article to be coated an anode in an electrolytic cell containing an aqueous acidic coating-forming electrolyte comprising predominantly aqueous sulfuric acid and passing an electric current through the electrolytic cell, including the anode, while maintaining the electrolyte at a temperature between its freezing point and 35 F. during the entire coating operation and simultaneously subjecting the electrolyte to agitation to maintain substantially uniform temperature conditions throughout the body of electrolyte during the coating operation.

4. A method of producing on the surface of a predominantly aluminum article an abrasionresistant aluminum oxide anodio coating by making the article to be coated an anode in an electrolytic cell containing an aqueous acidic coating-forming electrolyte comprising predominantly aqueous sulfuric acid and passing an electric current through the electrolytic cell, including the anode, while maintaining the electrolyte at a temperature of approximately 30 F. during the entire coating operation and simultaneously subjecting the electrolyte to agitation to maintain substantially uniform temperature conditions throughout the body of electrolyte during the coating operation.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Metal Finishing, February 1948, pages 65 thru 70.

The Anodic Oxidation of Aluminum and Its Alloys, by Jenny and Lewis (1940), pages 139 thru 145. 

1. A METHOD OF PRODUCING A HARD, ABRASION-RESISTANT ANODIC COATING ON THE SURFACE OF A PREDOMINANTLY ALUMINUM OR ALUMINUM ALLOY ARTICLE BY MAKING THE ARTICLE TO BE COATED AN ANODE IN AN ELECTROLYTIC CELL CONTAINING AN AQUEOUS ACIDIC COATING-FORMING ELECTROLYTE COMPRISING PREDOMINANTLY AN AQUEOUS SOLUTION OF SULFURIC ACID AND PASSING AN ELECTRIC CURRENT THROUGH THE ELECTROLYTIC CELL, INCLUDING THE ANODE, WHILE MAINTAINING THE ELECTROLYTE AT A TEMPERATURE BELOW 35* F. DURING THE ENTIRE COATING OPERATION. 